Electrostatic spinning of fibers was first described in U.S. Pat. No. 692,631. In principle, a droplet of polymer solution or melt is placed in a high electric field. The repulsion between the induced like-charges in the droplet compete with the surface tension of the liquid and when a sufficiently strong electric field is applied, typically 0.5-4 kV/cm, the electrostatic forces overcome the surface tension of the fluid and a jet of polymer solution or melt is ejected from the droplet. Electrostatic instability leads to rapid, chaotic whipping of the jet, leading in turn to fast evaporation of the solvent as well as stretching and thinning of the polymer fiber that is left behind. The formed fibers are then collected on a counter electrode, typically in the form of a nonwoven web. The collected fibers are usually quite uniform and can have fiber diameters of several micrometers, down to as low as 5 nm.
The three inherent properties of nano-fibrous materials that make them very attractive for numerous applications are their high specific surface area (surface area/unit mass), high aspect ratio (length/diameter) and their biomimicking potential. These properties lead to the potential application of electrospun fibres in such diverse fields as high-performance filters, absorbent textiles, fibre reinforced composites, biomedical textiles for wound dressings, tissue engineering scaffolding and drug-release materials, nano- and microelectronic devices, electromagnetic shielding, photovoltaic devices and high-performance electrodes, as well as a range of nano-fibre based sensors.
In many of these applications the alignment, or controlled orientation, of the electrospun fibres is of great importance and large-scale commercialisation of products can only become viable when sufficient control over fibre orientation is obtained at high production rates. In the past few years research groups around the world have been focusing their attention on obtaining electrospun fibres in the form of yarns of continuous single nano-fibres or uniaxial fibre bundles. Succeeding in this will allow the processing of nano-fibres by traditional textile processing methods like weaving, knitting and embroidery. This, in turn, will not only allow the significant commercialisation of several of the applications cited above, but will also open the door to many other exciting new applications. Incorporating nano-fibres into traditional textiles creates several opportunities. In the first instance, the replacement of only a small percentage of the fibres or yarns in a traditional textile fabric with yarns of similar diameter, but now made up of several thousands of nano-fibres, can significantly increase the toughness and specific surface area of the fabric without increasing its overall mass. Alternatively, the complete fabric can even be made from nano-fibre yarns. This has important implications in protective clothing applications, where lightweight, breathable fabrics with protection against extreme temperatures, ballistics, and chemical or biological agents are often required. On an aesthetic level, nano-fibre textiles also exhibit extremely soft handling characteristics and have been proposed for use in the production of artificial leather and artificial cashmere
Several processes for preparing yarns from electrostatically spun fibers are described in the scientific and patent literature. Some of the oldest processes date back to the 1930s and are described in seven US patents by Anton Formhals. In U.S. Pat. No. 1,975,504, fibres are electrostatically spun from a cogwheel spinneret onto the edge of a rotating wheel or endless belt setup. The fibers that collect on the edge of the wheel or belt are pushed or pulled off and twisted into yarns. A very similar process with multiple rotating collectors and re-collectors are also described in PCT Application WO 2005/123995 A1.
U.S. Pat. No. 2,187,306 describes a process by which a core-spun yarn can be made by electrostatically spinning fibers onto a pre-formed yarn or sliver of fibers, while U.S. Pat. No. 2,109,333 describes a process in which electrostatically spun staple fibers can be made into a yarn.
In U.S. Pat. No. 2,349,950, corona discharge from the counter-electrode in the electrostatic spinning setup is used to neutralize charged fibers before collecting them and twisting into yarns.
Ko et al. (Advanced Materials, 2003, 15, 1161-1165) described a process by which a “self-assembled” yarn can be made from polyacrylonitrile/carbon nanotube blends.
Processes have been described by Kim (PCT Application WO 2004/074559 A1), Smit et al. (Polymer, 46, 2005, 2419-2423) and Khil et al. (J. Biomed. Mater. Res. B 72 (2005) 117), in which yarns are made by drawing electrostatically spun fiber webs across a water bath surface and collecting the resulting bundled fiber yarns. In a variation by Ramakrishna at el. at Singapore National University, fibers are spun onto a water bath with a hole in the bottom of the bath. The vortex, formed by the water running out of the hole, twists and aligns the fiber webs into an aligned fiber bundle yarn that is collected in continuous fashion from the hole in the bottom of the bath.
PCT Application WO 2005/073442 A1 describes a process in which a nonwoven web of electrostatically spun fibers is cut into ribbons, or the web is spun in thin ribbon strips from the start. The nonwoven ribbons are then twisted using an air twister to form continuous yarns.
In PCT Application WO 2006/052039 A1 a special electrostatic spinning collector belt is described, consisting of an endless-belt type collector with grooves parallel to the machine direction and with conductive strips in the grooves on the belt. Fibers are spun onto the conductive strips in the grooves, and then removed in a continuous fashion and twisted into yarns.
Chinese patents CN 1766181, CN1687493 and CN 1776033 describe essentially the same process, wherein fibers are spun from opposing directions, using high voltage of opposite polarity. The fibers that collide mid-air between the spinnerets of opposing polarity are neutralized and then collected on take-up rollers and twisted into yarns.
In PCT Application WO 2006-135147 A1, Kim describes a special configuration in which a C-shaped nozzle block containing thousands of spinning nozzles is placed adjacent to a drum collector rotating with high linear velocity. Narrow webs of reasonably aligned fibres are collected off the rotating drum surface in a continuous fashion and twisted together to form a continuous yarn.
The ideal process for preparing continuous yarns from electrostatically spun fibres should be up-scalable, result in high degrees of fibre alignment and work for all polymers and/or polymer blends that can be electrostatically spun into fibres. Although the processes described in the prior art comply with some of these requirements to varying degrees, not one of the processes fully complies with all three requirements.
The yarns obtained from the various processes described in the prior art invariably suffer from one or more drawbacks. In many cases, for example the processes described by Formhals, the obtained yarns have very low or random degrees of alignment of fibres along the yarn axis. Alignment of fibres is very important for yarn strength since it ensures an optimally shared distribution of the tensile load between the fibres when the yarn is placed under tension. In other words, lower degrees of fibre alignment lead to lower strength yarns. Another drawback of some of the processes, like Ko's self-assembled yarn, is that it is difficult or costly to scale-up the process. Finally, in cases where fibres are electrospun onto a water bath, yarns cannot be made from water-soluble or water-sensitive polymers. This can be a major drawback if one considers that many of the biodegradable polymers for tissue engineering applications are water-sensitive.